WO2014099822A2 - Système et procédé d'identification de matériaux à l'aide d'une empreinte spectrale thz dans un milieu à haute teneur en eau - Google Patents

Système et procédé d'identification de matériaux à l'aide d'une empreinte spectrale thz dans un milieu à haute teneur en eau Download PDF

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WO2014099822A2
WO2014099822A2 PCT/US2013/075495 US2013075495W WO2014099822A2 WO 2014099822 A2 WO2014099822 A2 WO 2014099822A2 US 2013075495 W US2013075495 W US 2013075495W WO 2014099822 A2 WO2014099822 A2 WO 2014099822A2
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pulse
thz
signal
spectral
data
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WO2014099822A3 (fr
WO2014099822A4 (fr
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Patrick K. Brady
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Brady Patrick K
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation

Definitions

  • Embodiments of the present invention relate generally to structures and methods for detecting materials, including explosives or contraband, using a terahertz (THz) fingerprint.
  • materials including explosives or contraband
  • THz terahertz
  • THz imaging offers the promise of a nonintrusive technique to detect contraband hidden on the human body.
  • THz spectrometer systems have limited performance in a stand-alone detection mode of operation. Moreover, they are not able to penetrate high water content media, such as the human body. Even frequencies in the near infrared (NIR) band do not provide a solution because skin and outer layer materials (e.g., clothing) are likely to be opaque to infrared radiation.
  • NIR near infrared
  • THz detection systems rely on broad-band detectors to detect THz radiation. Typically, they require mechanically tunable interferometers to achieve spectral resolution. To achieve the required sensitivity, conventional THz detectors also require cooling to liquid helium temperatures. While THz spectrometer systems exist, they are too bulky and fragile for field applications.
  • detection/discrimination processes to identify and classify threats and to reduce false alarms.
  • a terahertz (THz) detector system implements a terahertz (THz) detector system.
  • Reflections from change in material boundaries and absorption spectra from non- biological material are detected by THz antenna arrays tuned to the known absorption frequencies of the material of interest, such as explosives or other contraband.
  • Spectral signatures or "fingerprints” are matched using these arrays in ensemble. Multiple spectral peak detection combined with application of signal processing algorithms enhances detection to reduce false positives (indicating the presence of a material of interest that is, in fact, not present) and lower false negatives (failing to indicate the presence of a material of interest that is, in fact, present).
  • a system to detect a material comprises a pulse generator to generate a terahertz pulse to excite molecules in the material, a detector to detect a signal having spectral components in the terahertz region emitted from the excitations, and a processor to perform statistical analysis and signal processing on the detected signal to reduce false positives and false negatives of detection.
  • a method for detecting a material comprises generating a pulse to excite molecules in the material, detecting a signal having spectral components in the terahertz region emitted from the excitations, and performing statistical analysis and signal processing on the detected signal to reduce false positives and false negatives of detection.
  • representing specific objects comprises simulating security screening scenarios, collecting THz spectral data representative of the objects, extracting features from the magnetic data that distinguish respective objects, and performing a pre-classification optimization method on the features.
  • Figure 1 is a schematic diagram of a system for detecting the presence of a material of interest using THz excitation pulses according to an embodiment of the present invention.
  • Figure 2 is a schematic diagram of an exemplary antenna-coupled diode
  • Figure 3 is a graph that shows the current-voltage (I-V) characteristics for two types of MIMs that can be used according to embodiments of the present invention.
  • Figure 4 is a graph showing the I-V characteristic and current density of a
  • Figure 5 is a set of coupled Maxwell-Bloch equations that can be used to calculate the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value for the maximum value.
  • Figure 6 illustrates simulated application of coherent bleaching to water in the
  • Figure 7 is a schematic diagram of an exemplary NIR amplifier laser system that can be used as a THz pulse source according to an embodiment.
  • Figure 8 is a schematic diagram of a laser-based THz system flow of THz current according to an embodiment.
  • Figure 9 is a schematic diagram of a two-stage SPFEL THz pulse source 900 according to an embodiment.
  • Figure 10 is a simulated image of an exemplary spiral nanoantenna designed for resonance in the mid-infrared frequency band, with an embedded tunnel diode according to an embodiment.
  • Figure 11 is a scanning electron microscope picture of an exemplary antenna array 1 100 according to an embodiment.
  • Figure 12 illustrates an exemplary THz-TDS system 1200 for characterizing polymer composites.
  • Figure 13 illustrates an exemplary THz waveform measured through air.
  • Figure 14 is a graph illustrating exemplary spectral power magnitude curves for an exemplary sample pulse and an exemplary reference pulse.
  • Figure 15 is a graphical representation of the refractive index calculated using both an iterative technique and an analytical approach.
  • Figure 16 is a graphical representation of the extinction coefficient determined using the iterative and analytical approaches.
  • Figure 17 is a graph that illustrates the absorption characteristic of a weak
  • THz pulse (2.3W/cm 2 ) as it propagates up to 15 absorption lengths through a coherent medium.
  • Figure 18 is a graph that illustrates the absorption characteristic of a high
  • Figure 19 is a graph that shows the peaks of the pulses illustrated in Figures
  • Molecules have unique vibrational and rotational frequencies in the Terahertz (THz) region. These unique spectral features in essence form a fingerprint of the molecules. Pulses in the THz range can be generated and used to excite these spectral features of molecules of a material of interest, such as an explosive or other contraband or material of interest. This fingerprint can be exploited to identify a particular material being surveyed. For example, the explosive material RDX exhibits spectral lines at 0.82, 1.05, 1.96, 2.20, and 3.08 THz. Simultaneous detection of several of these wavelengths in a material of interest being analyzed indicates a likely presence of the RDX material in the material of interest.
  • THz Terahertz
  • FIG. 1 is a schematic diagram of a system 100 for detecting the presence of a material of interest using THz excitation pulses according to an embodiment of the present invention.
  • a suspect target 102 which may contain a material of interest, is illuminated by an interrogation pulse 106 in the THz frequency range emitted by an interrogation pulse source 104.
  • interrogation pulse source 104 is a femtosecond THz pulse laser. An exemplary such laser is described in more detail below.
  • illumination of suspect target 102 with THz interrogation pulse 106 induces coherent bleaching.
  • Coherent bleaching reduces absorption of the pulse 106 propagating through the material being interrogated. This allows THz interrogation pulse 106 to penetrate to depths in the material being interrogated, particularly organic materials, such as explosives, or material with high water content, such as human tissue.
  • Illumination of suspect material 102 generates a reflected pulse 108.
  • Reflected pulse 108 is a modified version of THz interrogation pulse 106, which modifications are dependent upon, and indicative of, the molecular composition of suspect target 102. For example, reflected pulse 108 exhibits a spectrum
  • This spectral "fingerprint" can be used to identify materials of interest in suspect target 102, such as explosives or other contraband.
  • antenna/diode detector array 1 comprises a plurality of antennae (and/or diodes or other detection devices) that detect the spectral components of reflected pulse 108.
  • the antennae/diode detectors in antenna/diode array 1 10 are also referred to as "sensors".
  • antenna/diode detector array 1 10 comprises three antenna/diode detector subarrays 1 10a, 1 10b, and 1 10c.
  • Each antenna/diode detector subarray 110a, 110b, and 110c comprises one or more antennae that are tuned to a particular frequency.
  • detector antenna/diode arrays may comprise a plurality of antenna/diode subarrays with each antenna/diode subarray tuned to a frequency band.
  • detector antenna/diode arrays may comprise a plurality of antenna/diode subarrays with each antenna/diode subarray tuned to a frequency band.
  • Such an embodiment breaks up the frequency spectrum evenly for a more general purpose device (similar to a mass spectrometer).
  • antenna/diode detector array 110 is supplied to a data
  • Data acquisition and statistical analysis module 1 12 processes the output of antenna/diode detector array 110 to determine the spectral content of reflected pulse 108.
  • data acquisition and statistical analysis module 112 comprises one or more matched filters that are tuned to detect spectral components of interest.
  • the spectral components of interest are 0.8THz, 1.5THz, and 2.0THz. Consequently, data acquisition and statistical analysis module 1 12 would comprise three matched filters one tuned to detect 0.8THz, one tuned to detect 1.5THz, and one tuned to detect 2.0THz.
  • data acquisition and statistical analysis module 112 may include other processing to better detect the spectral content of antenna/diode detector array output 110, and thereby reduce false positives (indicating the presence of a material of interest that is not present) and false negatives (failing to indicate the presence of a material of interest that is present).
  • the system incorporates a series of detectors that perform single -point absorption peak measurements and matched-filter analysis. It does not require scanning of the full spectrum and FFT post processing of datasets. Each antenna is 'tuned' to a selective bandpass representing a unique spectral absorption peak of the explosive material.
  • Data analysis extracts unique spectral features, including: frequency of absorption peaks, peak widths, ratio between peak heights and peak slope to serve as discriminators of threat materials.
  • Data acquisition and statistical analysis module 1 12 provides its output to a pattern classification module 114.
  • Pattern classification module 1 14 comprises a database of spectral content of the reflected pulse of one or more materials of interest when interrogated by a THz interrogation pulse. Patter classification module 1 14 compares the spectral component output from data acquisition and statistical analysis module 112 with the spectral content of one or more of the materials stored in its database. Pattern classification algorithms match spectra to a database of validated spectra. Pattern classification module 114 can provide an alert or warning if there is a match. The alert can be audible, visual, or any other way to provide an alert that a material of interest, such as an explosive or other contraband was detected. [0038] Detection of frequencies in the THz band requires new photonic and electronic devices suited to the THz band.
  • a femtosecond pulse THz laser drive source is used as THz interrogation source 102.
  • THz interrogation source 102 a femtosecond pulse THz laser drive source penetrates high water content materials based on the phenomenon of coherent bleaching that overcomes the limitations of conventional THz detectors.
  • antenna/diode detector array 110 comprises
  • ACT devices comprise nanoantenna structures combined with resonant tunneling Metal Insulator Metal diodes (MIM).
  • the nanoantenna structure comprises Metal Insulator Insulator Metal diodes (MUM).
  • MIM Metal Insulator Metal diodes
  • MUM Metal Insulator Insulator Metal diodes
  • Figure 2 is a schematic diagram of an exemplary antenna-coupled diode
  • ACT device 200 comprises an antenna 202, a diode 204, and a DC filter 206.
  • Antenna 202 resonates in the presence of energy to which it is tuned, for example, energy in the THz frequency range, and passes energy to diode 204.
  • Diode 204 converts the energy generated by antenna 202 to direct current (DC).
  • diode 204 is a MIM diode. In another embodiment, diode 204 is a MUM diode or other
  • ACT device 200 can be coupled to a load 208. Consequently, ACT device 200 delivers current to load 208 when in the presence of radiation to which it is tuned.
  • Various circuits such as analog to digital convertors or digital logic circuits may be substituted for load circuit 208 depending upon the application and data acquisition system. Such circuits would be well known to those skilled in the art.
  • ACT detectors such as ACT detector 200, are used in a THz spectroscopy system because they exhibit an excellent noise- equivalent power ( EP), a very fast response, and can operate at room temperature.
  • Figure 3 is a graph that shows the current-voltage (I-V) characteristics for two types of MIMs that can be used in embodiments.
  • Curve 1702 corresponds to a Cr/C ⁇ CVCr MIM diode.
  • Curve 1704 corresponds to a b/Nb 2 0 5 /Nb MIM diode.
  • the Cr/C ⁇ CVCr diodes can be fabricated with bowtie or other antennas to form ACT devices to use in embodiments.
  • Lower impedance MIM diodes such as those with a NiO insulator and Ni electrodes, may provide an improved impedance match with the antenna (such as antenna 202), due to the lower barrier heights of such materials.
  • the improved impedance matching of such a MIM device may come at the cost of reduced responsiveness (DC current out per unit AC power in) that results from the lower diode nonlinearity of low barrier-height devices.
  • the desire for both low impedance and high responsivity indicates using a double-insulator MUM diode instead of a single insulator device.
  • the additional insulator layer increases diode nonlinearity without increasing the diode impedance.
  • Figure 4 is a graph showing the I-V characteristic and current density of a
  • Curve 402 shows current density for a MUM device according to an embodiment. Curve 402 illustrates that such a MUM device exhibits both high nonlinearity and low resistance. Such a device can be used at frequencies greater than 1 THz.
  • preferred ACT devices use a double-insulator diode (MUM). Double insulator diodes provide a greater electrical nonlinearity - and therefore improved responsiveness and NEP - than single insulator MIM diodes.
  • Other devices such as microbolometers, HgCdTe, and Schottky diodes, can be used in embodiments. While such devices may not be as suitable as MIIM-diode embodiments, they may be used in embodiments where less sensitivity and speed are required.
  • Embodiments generate interrogating THz pulses to stimulate a process known as coherent bleaching.
  • Coherent bleaching occurs when a medium in which a pulse travels can store energy momentarily before returning it to the pulse via stimulated emission.
  • a pulse with enough energy to cause a state inversion can exhibit coherent bleaching over relatively large distances if its temporal width is shorter than or comparable to the dephasing time of the medium in which it is traveling.
  • SIT Self- Induced Transparency
  • Figure 5 is a set of coupled Maxwell-Bloch equations that can be used to model this region of coherent bleaching in a material.
  • ⁇ represents the dipole moment of the medium
  • E is the incident electric field
  • p 3 o is the initial state of inversion for the population
  • pi, p 2 , and p 3 represent the dispersive (in-phase) polarization, the absorptive (in-quadrature) polarization, and the inversion parameter of the two-level system approximation of the medium, respectively.
  • NMR nuclear magnetic resonance
  • GPR ground penetrating radar
  • Figure 6 illustrates simulated application of coherent bleaching to water in the
  • Curve 602 corresponds to W2866, spacer 12 ⁇ .
  • Curve 604 corresponds to W2968, spacer 18 ⁇ .
  • FIG. 7 is a schematic diagram of an exemplary NIR amplifier laser system 700 that can be used as a THz pulse source according to an embodiment.
  • a laser 702 delivers its energy through a series of lenses (704, 708, 712, 716), mirrors (710, 714) and diffraction grating (706) to a nonlinear medium 718 to generate THz waves from the NIR laser's output energy.
  • Such a system can be used as THz interrogation pulse source 104 in Figure 1 in an embodiment.
  • laser 702 produces ⁇ 100-fs, 4-mJ NIR pulses. Using such a laser provides THz peak power exceeding 1 TW/cm .
  • optical rectification is performed by generating a THz pulse using a near infrared (NIR) pulse laser 702.
  • NIR near infrared
  • the pulse generated by NIR laser 702 impinges a nonlinear medium 718 to induce a short polarization that has the form of the laser-pulse envelope.
  • the rectified pulses have a frequency content extending into the THz frequency range.
  • the efficiency of the optical rectification process depends on the properties of nonlinear-medium 718, the phase-matched length of the nonlinear medium 718, and the Intensity, I, of the laser 702.
  • Nonlinear media including Zinc Telluride (ZnTe), Gallium Phosphite (GaP) and Lithium Niobate (LiNb0 3 ) can be used in embodiments as nonlinear medium 718.
  • ZnTe Zinc Telluride
  • GaP Gallium Phosphite
  • LiNb0 3 Lithium Niobate
  • nonlinear medium 718 has a group refractive index for the femtosecond laser pulse that is equal to the phase refractive index for the THz pulse.
  • the optical group and THz phase indexes are such that the THz pulse lags behind the NIR pulse. This limitation can be overcome using a laser pulse with an intensity front that is tilted with respect to its propagation direction.
  • FIGs 7 and 8 illustrate an exemplary system 700 for generating a THz band pulse according to an embodiment.
  • system 700 can be used to generate a THz interrogation pulse source 104 in an embodiment.
  • a NIR fs laser 702 generates a pulse that impinges a diffraction grating 706 through a focusing lens 704.
  • the portion of the pulse reflected off diffraction grating 706 is directed through a lens 708 to a mirror 710.
  • Mirror 710 directs the pulse through another lens 712 to a mirror 714.
  • Mirror 714 directs the pulse to nonlinear medium 718 through focusing lens 716. After propagating through nonlinear medium 718, the pulse emerges as THz pulse 720.
  • nonlinear medium 718 is a wedge LiNb03 crystal. High NIR-to-THz efficiencies have been measured from this type of crystal.
  • NIR laser 702 produces ⁇ 100-fs, 4-mJ NIR pulses resulting in an anticipated THz peak power close to 200 MW. Focusing this THz pulse with a silicon lens to a sub micron size results in a THz-pulse peak intensity 1.1 TW/cm .
  • laser system 700 also incorporates an acousto-optics dispersive filter that provides control over the NIR pulse spectrum. Such a filter can be used to tailor the shape and spectrum of the generated THz pulse. This feature might prove useful for exploring the frequency response of the developed detector or mimicking detection scenarios.
  • a IR pulse source 802 emits pulses toward a diffraction grating 804.
  • Diffraction grating 804 redirects the pulses through a lens 808 and optical directional elements 806 and 810 to a nonlinear medium 812. THz pulses are emitted from nonlinear medium 814 as described above.
  • THz interrogation pulse source 104 generates THz pulses using an accelerator-based technique.
  • an accelerator-based technique For example, in an embodiment, a variant of the Smith-Purcell free-electron laser (SPFEL) is used.
  • SPFEL Smith-Purcell free-electron laser
  • the size of a SPFEL device allows for a portable implementation in an embodiment.
  • FIG. 9 is a schematic diagram of a two-stage SPFEL THz pulse source 900 according to an embodiment.
  • Pulse source 900 can be used as THz interrogation pulse source 104 in an embodiment.
  • particles emitted from a particle source 906 travel past gratings 908 to a particle sink 910.
  • gratings 908 and substrate 912 are comprised of metallic boundaries.
  • a low energy ( ⁇ 100 keV) DC sheet electron beam propagates above a periodic metallic structure (e.g., a grating 908 in radiating stage 904) and radiates spontaneous Smith-Purcell radiation.
  • the electromagnetic fields associated with some spectral orders of the radiation are evanescent and are confined near the structure surface. These are called “surface modes". Some of these surface modes interact with the electron bunch and thereby impart an energy modulation along the bunch with a period corresponding to the strongest surface mode. Because of the low electron velocity, this energy modulation turns into a density modulation at the wavelength of the strongest evanescent mode. This "micro-bunching" of the electron bunch results in an enhancement of emission at a wavelength comparable to and larger than the micro- bunching period.
  • the SPFEL is operated in a continuous wave (CW) mode thereby producing narrow-band THz pulses.
  • the THz pulses generated by a two-stage SPFEL THz pulse source produce an average power on the order of Watts. Additional modes of operation may produce short (broadband) THz pulses.
  • the electron bunch is compressed at the sub-picosecond level.
  • the timing of the electric field in the cavity in launching stage 902 is such that the center of the bunch experiences no accelerating field, while the tail of the electron bunch experiences an acceleration, and the head of the electron bunch experiences a deceleration. Because of the sub-relativistic nature of the ⁇ 100-keV beam, the tail eventually (after a small drift) catches up to the head resulting in an electron-beam bunching.
  • a 30-fs electron bunch is generated.
  • the footprint of SPFEL THz source 900 is on the order of 0.5x0.5x0.5 m 3 .
  • THz source 104 Coherent bleaching requires THz source 104 to create an extremely short pulse of high peak power and a wide band THz signal.
  • Current THz source technologies include: solid state oscillators, quantum cascade lasers, optically pumped solid state devices, free electron lasers (FEL) and others. As described above, therefore, preferably THz source 104 is a pulsed- laser driven THz emitter or a FEL device.
  • FEL devices include Klystrons, Travelling Wave Tubes, Backward
  • Oscillators, Gyrotrons and Smith-Purcell FEL devices While laser-based sources are usable, a bench scale (smaller footprint) device is preferable.
  • a bench scale device in an embodiment, the Metal Grating FEL based on the Smith-Purcell effect, described above with respect to Figure 9, is used.
  • THz pulses including but not limited to: optically pumped THz laser, optical rectification, photoconductive dipole antennae, backward wave oscillator, frequency mixing, and conventional free-electron lasers.
  • Table 1 the main features of the most popular among these techniques are compared with the characteristics preferred 'Smith-Purcell free-electron laser' (SPFEL) that is described above with respect to
  • Table 1 Summary of popular coherent THz radiation sources. The last column corresponds to the anticipated performance of the
  • optical rectification or photoconductive dipole switch devices are most popular. In both types of devices, a femtosecond-class laser is used to drive the source.
  • the laser impinges an electro-optical crystal and thereby generates broadband coherent THz radiation.
  • a split antenna is fabricated on a semiconductor substrate to create a switch.
  • a DC bias is placed across the antenna, and the femtosecond laser is focused in the gap in the antenna.
  • the bias-laser pulse combination allows electrons to jump the gap rapidly, and the resulting current in the antenna produces a terahertz electromagnetic wave. Both of these techniques create THz pulses with duration of - 100 fs; these pulses are always broadband (typically 0.5 THz).
  • the nanoantennas in the ACT devices used in antenna array 1 10 can be made using frequency selective surfaces (FSS). See, e.g., D. Kotter, S. Novack, W. Slafer, P. Pinhero "Theory and Manufacturing Processes of Solar Nanoantenna Electromagnetic Collectors", The Journal of Solar Energy Engineering, Feb 2010, Vol. 132 and B. Monacelli, J. Pryor, B. A. Munk, D. Kotter, and G. D. Boreman, "Infrared frequency selective surface based on circuit-analog square loop design," IEEE Transactions on Antennas and Propagation, vol. 53, no. 2, pp.
  • FSS frequency selective surfaces
  • FIG. 10 is a simulated image of an exemplary spiral nanoantenna 1000 designed for resonance in the mid-infrared frequency band, with an embedded tunnel diode 1003 according to an embodiment.
  • Other antenna types may be applicable according to an embodiment.
  • Device resonance in antenna 1000 is caused by the wave like properties of electromagnetic radiation absorbed by a metal antenna.
  • the basic theory of operation is as follows.
  • the incident electromagnetic radiation (flux) from a THz or thermal source excites plane-wave electrical currents in microantenna structure 1000.
  • antenna 1000 When antenna 1000 is excited into a resonance mode it induces the plasma release of electrons from the metal antenna arms 1002.
  • the electrons freely flow along the antenna generating current at the same frequency as the resonance.
  • Figure 10 also shows that current flows toward the antenna feed point.
  • the electric field is clearly concentrated at the center feedpoint. This provides a convenience point to collect energy.
  • Figure 2 also demonstrates the ability to embed a diode into the feedpoint of the antenna structure for rectification and signal processing.
  • rectification is performed using a metal- insulator- metal (MIM) tunneling diode.
  • MIM metal- insulator- metal
  • Other diodes, such as MUM diodes can be used in embodiments, and may provide better performance.
  • diodes require highly nonlinear current-voltage characteristics, sharp turn-on, and very low dark current. In an embodiment, these requirements are achieved through use of asymmetric diode contact metals and energy band engineering of the tunneling insulator layers.
  • insulators are used as they can provide additional non-linearity over single insulator MIM diodes.
  • the multiple insulators (MUM) induce resonant structures that support high-energy tunneling.
  • the devices also may exhibit a negative differential resistance (I-V) curve behavior.
  • negative differential mobility is induced in the MUM diodes. Diodes with negative resistance enable the implementation of solid-state oscillator circuits in the 0.1 - 2 THz region. In turn, this provides increased sensitivity of detection as well as low power, compact sources for active interrogation.
  • FIG. 11 is a scanning electron microscope picture of an exemplary antenna array 1 100 according to an embodiment. Although not illustrated, the array can be ganged into subarrays of multiple antenna elements to optimize reception of a THz signal and to improve SNR.
  • antenna array 1 100 discrete antennas, such as antennas 1102a, 1102b, and
  • a frequency selective service FSS
  • Resonant frequency and spectral bandwidth of the FSS are controlled by the geometry of the periodic metallic antenna structures and the refractive index of associated thin film materials.
  • the FSS acts like a physical template that passes only electromagnetic energy representative of a particular material of interest, such as an explosive. It also provides cold filter rejection of broadband thermal background noise.
  • the use of a FSS nanoantenna array provides unprecedented narrow band THz discrimination with tunable selectivity to absorption lines of pre-defined chemical species. This enables positive identification of threats and greatly reduces false negatives.
  • detection of a material of interest occurs by detecting THz excitation pulses reflected from an object under surveillance.
  • signal scattering is likely to occur that reduces coherence.
  • the detector is configured as a phased array to increase the field-of-view and to maximize capture of the retro-reflected signal.
  • the pulse reflected from the illuminated material is modified due to resonant material absorption.
  • the modified reflected pulse appears to be "ringing." This ringing effect arises from a number of factors including: absorption in the atmosphere, propagation effects from a skin boundary, propagation effects from a media/tissue boundary, and absorption from materials of interest, such as explosives.
  • this modification of the interrogating THz pulse serves as a "fingerprint" that can be used to identify the material under investigation.
  • a series of FSS detectors perform single-point absorption peak measurements and matched- filter analysis of the reflected signal generated by the interrogation signal. As a result, scanning of the full spectrum and FFT post processing of datasets are not required.
  • Each antenna in the nanoantenna array such as nanoantenna array 1000, is "tuned” to a selective band pass that represents a unique spectral absorption peak of the explosive material, or other contraband.
  • embodiments allow real time response and multiple measurements in a variety of acquisition modes including passive scanning.
  • data analysis is used to extract unique spectral features, including: frequency of absorption peaks, peak widths, ratio between peak heights and peak slope. These features serve as discriminators or "fingerprints" of materials under investigation.
  • pattern classification algorithms match spectra to a database of validated spectra. Matches indicate a presence of the matched material.
  • THz device components are fabricated using polymer composites.
  • component fabrication relies on characterization of the polymer composites using a method to characterize the spectral properties of the media.
  • An exemplary way to characterize the properties of the media is to use THz time domain spectroscopy (THz-TDS). THz-TDS has been widely used to extract frequency dependent optical properties in the THz region.
  • FIG. 12 illustrates an exemplary THz-TDS system 1200 for characterizing polymer composites.
  • time delay 1206 moves it changes the position of the THz waveform with respect to a probe beam pulse 1215 as they both arrive at the crystal (ZnTe) 1212.
  • the entire THz waveform is extracted by moving the time delay while collecting THz electric field data at every position surrounding the pulse.
  • a Ti:Sapphire laser source provides a pulse through a half-wave plate 1201 to a beam splitter 1204.
  • Beam splitter 1204 splits the pulse signals and directs a portion to a time delay 1206 and a portion to a quarter-wave plate 1204.
  • Pulse 1215 goes through lens 1216 to a ZnTe crystal detector 1217 through a pellicle 1219.
  • Time shift 1206 adds delay to the pulse path. Incremental delay over the entire time from of the pulse allows collection of THz field data at each incremental point around the THz pulse.
  • the time shifted pulse is passed through a lens 1208 to a photoconductive antenna 1210.
  • Photoconductive antenna 1210 emits a THz pulse 1212 that is directed by parabolic mirrors 121 la and 1211b through a transmission sample 1213.
  • Transmission sample 1213 is a sample of the nonlinear material whose optical properties are desired to be characterized.
  • THz pulse 1212 is then directed by parabolic mirrors 1211c and 121 Id to pellicle 1219, which reflects THz pulse 1212 to detector crystal 1217.
  • THz pulse 1212 alters the birefringence properties of detector crystal 1217, which, in turn, alters the phase characteristics of co-propagating pulse 1215.
  • a Wollaston prism 1218 separates pulse 1215 into its orthogonal components and directs each of the orthogonal components to one of a pair of balanced diodes 1220.
  • Figure 13 illustrates an exemplary THz waveform 1302 measured through air.
  • Waveform 1302 represents a THz pulse traveling through air with no sample in the pulse's path. As such, waveform 1302 can be used as a baseline or reference for determining the response of the THz pulse when interrogating a material of interest.
  • FIG. 14 is a graph illustrating exemplary spectral power magnitude curves for an exemplary sample pulse (curve 1404) and an exemplary reference pulse (curve 1402).
  • the refractive index and extinction coefficients can be calculated analytically only if the absorptive loss of the sample is small enough to make k negligible for portions of the optical transfer function.
  • a more precise approach is used.
  • the more precise approach known to those skilled in the art, involves breaking down the complex equation into real and imaginary parts and solving a binary equation at each frequency point.
  • the resolution for the complex refractive index needs to be large to again help ensure modeling precision. This requires iteratively solving binary systems of equations.
  • the difference in refractive index and extinction coefficients solved by the simplified analytical method and the iterative method can be seen from Figures 15 and 16.
  • Figure 15 is a graphical representation of the refractive index calculated using both the iterative technique and the analytical approach.
  • Figure 16 is a graphical representation of the extinction coefficient determined using the iterative and analytical approaches. As shown in Figures 15 and 16, both the refractive index and the extinction coefficient change appreciably when calculated by the more accurate iterative technique.
  • curve 1502 is calculated using the iterative approach and curve 1504 is calculated using the analytical approach.
  • curve 1602 is calculated using the iterative approach and curve 1604 is calculated using the analytical approach.
  • THz detection system includes that the response is not thermal time-constant limited as compared to bulk bolometer detectors.
  • Laser lk/s duty cycle and lps ACT detector response sustains less than lOsec interrogation rates and achieves an order of magnitude improvement in SNR.
  • a system according to an embodiment provides inherent polarization selectivity of antenna-based detectors.
  • Antenna geometry dictates the preferential polarization orientation of detector sensitivity. Bulky and expensive optics are not required to detect specific polarization of incident radiation.
  • a detector provides for multi-band operation.
  • the detector uses discrete, tuned resonant antenna elements.
  • Arrays of multiple antenna sub-arrays can be configured to concurrently detect multiple spectral features (resonant peaks) of explosives or other material of interest. This greatly increases system response time and reduces false alarms.
  • an exemplary graphical representation 1 lOd illustrates THz spectral peaks of interest output or registered by sensors in antenna/diode detector array 1 10.
  • the spectral data represents the possible presence of explosive materials within the field-of-view of the THz Imaging System 100 due to the spectral energy at 2.0THz.
  • the shape of the response curve 1 lOd representing spectral peaks of explosive material can be further modulated by several factors including noise introduced by propagation of interrogation pulse 106 through various mediums and impacts of high water content within the object under interrogation 108.
  • noise introduced by propagation of interrogation pulse 106 through various mediums and impacts of high water content within the object under interrogation 108.
  • advanced data analysis methods are required to filter and compensate for noise and to extract out unique features representative of threat materials.
  • Exemplary data analysis methods comprise detection and classification
  • An exemplary data analysis method is termed the feature extraction method.
  • the feature extraction method extracts features from the output of antenna/diode detector array 1 10 that are unique to materials of interest, such as explosives. Examples of extracted features, include: 1) a spectral fingerprint in the frequency domain for each sensor; 2) a total power value of the signal detected by each sensor; and 3) time domain data for each spectral fingerprint that represents peak rise time, peak shape/slope, and time correlation to the interrogation pulse.
  • data acquisition and statistical analysis module 112 performs data acquisition using analog to digital conversion hardware such as sample and hold circuitry, ADC converters, phase-lock loops, PLL and other methods apparent to those skill in the art. In an embodiment, all analysis of spectral peaks is performed using digitized data.
  • a point-by-point Fast Fourier Transform is
  • the FFT computation provides FFT values for each sensor as a function of frequency (in the frequency domain). Still referring to Fig. 1, data acquisition and statistical analysis module 112, "power in the signal" is computed for each sensor using the reverted FFT values. This computation is performed by determining the area under the response curve. That is, integrating the function of the response curve.
  • data acquisition and statistical analysis module 1 12 identifies a portion of time within the period of time that represents when the peak value of 1 lOd occurs and correlates it to interrogation pulse event 104.
  • pattern classification module 1 14 uses a fusion of methods to perform near-real time identification of threats.
  • the following exemplary analysis methods are fused: i) time domain feature extraction; ii) wavelet analysis; iii) matched filter detection; and iv) model-based frequency analysis.
  • the values derived or computed from these exemplary analysis methods are further processed, for example using fuzzy logic, to increase the probability of accurate classification.
  • Pattern classification methods begin by gathering raw sensor data, for example, from security screening system 100.
  • Raw data is obtained from sensors 110 and represented as spectral waveforms. The data is acquired at user selectable sample rates.
  • Sensor-level digital signal processor (DSP) firmware extracts features and patterns from the raw spectral data. Additionally, the extracted features and magnetic patterns can be further post-processed by a standard desktop or laptop computer using custom software.
  • DSP digital signal processor
  • the extracted features and patterns from each sensor of security screening system 100 are analyzed as groups to provide additional information, such as multiple absorption peaks representative of a particular type of explosive and/or ratio of peak amplitudes representative of explosives.
  • features or “feature extraction” is defined as “repeatable characteristics in the raw data that are consistent for the same group or class of detectable threat objects.”
  • the features are available in the time domain, the frequency domain, and the two frequencies combined, that is, the time/frequency domain.
  • a pattern classification method uses a fusion of classification analytical methods to improve the signal-to- noise performance of sensors and to extract unique spectral features.
  • Specific classification analysis methods include: wavelets, matched filters and model-based frequency analysis.
  • One exemplary embodiment of a classification analysis method according to the invention is a wavelet method.
  • the wavelet method provides the means to extract secondary or complex spectral features.
  • the exemplary wavelet method allows the simultaneous extraction of both primary resonant frequencies and secondary harmonic signals having different frequency resolutions. Additionally, the wavelet method preserves the timing information (time domain) of the signal that other data analysis methods fail to maintain.
  • the wavelet method is dependent upon deriving a waveform transform that best matches the signal characteristics of the object being analyzed (for example, TNT explosive).
  • the function of the wavelet method provides a "best fit" of the wavelet to the pertinent portions of the theoretical absorption features of materials that comprise the threat object.
  • Wavelets derived from the wavelet method are well suited for the analysis of predominately non-stationary signals that have sudden spikes or peak values and a transient existence.
  • the wavelet method uses wavelets for feature extraction. That is, the numerical implementation of the wavelet transform is a filter bank designed for processing of signals that have a short duration (transient).
  • the wavelet transform uses a correlation operation to compare real-time signals to an elementary function. The wavelet transform compares the response signal to a pre-defined set of short waveforms called the fundamental wavelet (or mother wavelet).
  • the wavelets have different time durations, or scales, that mathematically represent impulse-like functions. This enables near real-time processing of impulse signals, such as induced coherent bleaching effects.
  • the wavelet transforms of the wavelet method indicate the frequency of the signal and indicates the timing of when the frequency occurs. That is, the wavelet method applies wavelets to characterize a signature simultaneously in both the time and frequency domains. Accordingly, the wavelets are used to:
  • Typical detrending techniques use low-pass filters, which can also impact or alter desired signal features.
  • wavelet-based detrending preserves the important features of the original signal.
  • the signature of a potential threat object is acquired, such as, during the period of time a person/item is at a security checkpoint.
  • various materials of interest such as explosive, generate a unique spectral signature or response
  • other benign materials also have overlapping spectral signatures.
  • the uniqueness is not readily apparent with analysis methods which use only one basis function (complex sinusoidal).
  • the wavelet method reduces to practice a series of "mother wavelets" that are tailored to match the threat signals of interest.
  • Another exemplary embodiment of a classification analysis method according to the invention comprises a matched filter method.
  • the matched filter method provides a technique to filter out typical "false alarm" noise responses.
  • the matched filter method looks at measured spatial data. The results are compared to modeled data.
  • a matched filter can be used to "match" a particular transit spectral waveform to achieve the maximum signal to noise ratio (SNR) and to emphasize certain signal bands where high fidelity information is present while deemphasizing regions that are more prone to noise corruption.
  • SNR signal to noise ratio
  • a classification analysis method is a model-based frequency analysis method (super-resolution). Often important features of the spectral response are not evident in the time waveform of the signal. Therefore, the resolution of this frequency response is difficult to resolve and to acquire the important features of the responses for additional processing and/or analysis.
  • the model-based frequency analysis method is a model-based analysis technique to improve resolution, and includes using the following various methods: matrix pencil, covariance, prony's method, and principle component auto-regressive (PCAR).
  • the frequency analysis method is based on the "matrix pencil" method.
  • the matrix pencil method uses the matrix pencil method, the magnetic response will be modeled as a time series and approximated with a recursive difference equation.
  • the mathematical equations and derivative of the matrix-pencil method would be well known to those skilled in the art.
  • the matrix pencil method quantifies the resonance frequency and the primary frequencies where the power of the signal resides.
  • the matrix pencil method is also optimized to obtain a super-resolution power spectra even when only small data sets are available.
  • Fuzzy logic is a problem-solving control system methodology and is implemented, for example, by software. Fuzzy logic provides a simple way to arrive at a definite conclusion based upon sometimes vague, ambiguous, imprecise, noisy, or missing input information. Fuzzy logic methodology is an approach to control problems by mimicking how a person would make decisions, only much faster. Fuzzy logic incorporates a simple, rule-based, IF X AND Y THEN Z, approach to solving a control problem rather than attempting to model a system mathematically. The fuzzy logic model is empirically- based relying on an historical knowledge and experience.
  • Fuzzy logic can process nonlinear systems that would be difficult or impossible to model mathematically.
  • An exemplary fuzzy logic explores relationships between multiple data inputs to reach empirical conclusions. For example, in one embodiment, fuzzy logic is used to assign different weights to spectral features such as slope, rise time, magnitude, and peak shape of the spectral response. Fuzzy logic is also used to weigh the confidence level of the classification decision.
  • raw sensor data is outputted from respective sensors
  • the empirical conclusions are inputted from fuzzy logic to a neural network for further processing.
  • Each of the extracted spectral features or characteristics are assigned weight values by the probabilistic neural network.
  • the neural network compares the real-time spectral response to a prior trained database of known threat signature features. False alarms are greatly reduced and potential threats are identified and assigned a level of probability of being a threat.
  • a detector according to an embodiment can operate in an uncooled
  • antenna/diode detector array 1 10 functions as an imaging device.
  • the 'core' nanoantenna technology is scaled to implement imaging applications.
  • each antenna/diode element in antenna/diode detector array 1 10 serves as a pixel in an imaging array.
  • a 320x240 pixel array, with associated data acquisition hardware (or firmware or software) can be used to digitize a terahertz image.
  • the response time of the rectifier is more than adequate to sustain a 30Hz frame rate that is required to provide realtime imaging.
  • Figure 17 is a graph that illustrates the absorption characteristic of a weak pulse (2.3W/cm 2 ) as it propagates up to 15 absorption lengths through a coherent medium.
  • Figure 18 is a graph that illustrates the absorption characteristic of a high
  • THz pulse peak intensity 760 x 10 W/cm
  • the high power of the pulse in a short time compared to the medium coherence time inverts resonance such that absorption is reduced to almost 0.
  • Figure 19 is a graph that shows the peaks of the pulses illustrated in Figures 17 and 18 plotted as a function of distance.
  • curve 1902 corresponds to the pulse propagation absorption profile without coherent bleaching shown in Figure 17
  • curve 1904 corresponds to the pulse propagation absorption profile with coherent bleaching shown in Figure 18.
  • FIG. 6 An exemplary absorption profile for a high-power short THz pulse is shown in Figure 6.
  • the pulse response illustrated in Figure 6 is for a pulse having a peak intensity of 760 x 10 W/cm .
  • the high power within a time shorter than the coherence time inverts resonance such that the absorption is reduced almost to zero. This is the result of coherent bleaching.
  • absorption is reduced accordingly.
  • mammal muscle is approximately 75% water.
  • the signal reflected from a specimen requires measuring the round-trip signal. With 0.36% one-way transmission, the square of that (lxlO 5 ) is the round- trip signal. Assuming an IED reflectivity ⁇ 10%, the exit signal should be ⁇ 10 "6 of the incident intensity. For example, for a 10 12 W/cm 2 signal, the exit transmission intensity would be ⁇ 10 6 W/cm 2 .
  • the exponential scattering coefficient of animal tissue is between 0.05 cm “1 and 2.13 cm “1 for frequencies between 0.02 and 2 THz.
  • the lower limit is negligible compared to the bleached absorption coefficient, but the upper limit is not.
  • scattering may provide an unsaturable linear baseline loss (unless coherent scatter occurs). This might reduce the two-way transmission to as little as 10 "10 of the incident signal making transmission a more favorable method for detection.

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Abstract

L'invention concerne un détecteur de matériau qui comprend un générateur d'impulsions pour générer des impulsions afin d'exciter des molécules dans le matériau et un détecteur pour détecter un signal généré à partir des molécules excitées dans la région des térahertz. Des caractéristiques spectrales dans le matériau sont analysées pour identifier le matériau. Une détection peut être réalisée à l'aide d'une structure de réseau de nano-antennes ayant des antennes ajustées pour détecter l'émission spectrale attendue. Le réseau de nano-antennes peut comprendre des antennes ayant des diodes MIM ou MUM. Un traitement de signal et une analyse statistique sont utilisés pour réduire des faux positifs et des faux négatifs dans l'identification du matériau.
PCT/US2013/075495 2012-12-17 2013-12-16 Système et procédé d'identification de matériaux à l'aide d'une empreinte spectrale thz dans un milieu à haute teneur en eau WO2014099822A2 (fr)

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